The lift of a helicopter is provided by the main rotor via its collective pitch position. In the present application, the term “collective pitch” refers to the collective pitch of the blades of the main rotor, unless specified otherwise.
The collective pitch of the blades of the main rotor is measured at the collective pitch control; this control comes from an instruction applied by the pilot to the collective pitch control member, however this instruction might be corrected by an automatic pilot type apparatus as a function of other parameters; there is no strict equivalence between the position of the pitch control member for the blades of the main rotor and the actual collective pitch of those blades.
The method and apparatus of the invention make priority use of information corresponding to the actual collective pitch of the main rotor; nevertheless, it is possible to make use of information relating to the position of the member for controlling the pitch of the main rotor, in particular data or a signal delivered by a potentiometer type sensor that is sensitive to the position of the pilot's collective pitch stick.
The tail rotor enables helicopter movement about the yaw axis to be controlled by performing two essential functions: the pilot-control function about the yaw axis; and the anti-torque function. The pilot-control function about the yaw axis enables the pilot to control directly and dynamically the behavior of the helicopter about its yaw axis by acting on the steering pedals to control turning, side slip, and/or lateral acceleration.
The purpose of the anti-torque function is to limit disturbances about the yaw axis whenever there is a change in the collective pitch.
In order to vary the lift of a helicopter, the angle of incidence of the blades of the main rotor is modified via the collective pitch control. This modification causes variations in the torque exerted by the main rotor on the helicopter. Without any correcting action, this variation in torque induces an effect about the yaw axis of the machine: turning or side slip. In order to mitigate that drawback, the anti-torque function automatically adjusts the control applied to the tail rotor as a function of variations in the collective pitch control.
This adjustment is implemented by positioning (and/or determining) the blade pitch variation control that is delivered to the tail rotor as a function of the position (or value) of the collective pitch, in application of a predefined relationship.
The pilot instruction generated by the pilot-control function varies around this static position or “neutral point” that results from the anti-torque function; the pitch of the tail rotor is controlled in a manner that is not necessarily symmetrical nor centered.
On a light helicopter, these two functions are generally provided by the pilot who actuates the tail rotor control pedals—or steering pedals—or other equivalent member for this purpose. Instructions from the pilot are optionally associated with correction instructions from the flight control system (automatic pilot or electrical flight control system).
On heavy helicopters (e.g. 9 (metric) tonnes or more), variations in torque on changes of collective pitch are large and generate strong disturbances about the yaw axis. The anti-torque function requires high levels of yaw control that cannot be handled directly from the steering pedals (see operational constraints specified below). It is therefore necessary to separate these two functions by providing a specific apparatus enabling the anti-torque function to be handled automatically.
Implementing these two functions is generally done as follows:
the anti-torque function is implemented by a mechanical decoupling box which arbitrarily applies a pitch variation control to the tail rotor that is proportional to the collective pitch applied to the main rotor;
the function of pilot control about the yaw axis is performed directly by the pilot acting on the pedals; the pilot instruction is associated with additional instructions from the flight control system (automatic pilot or electrical flight control system).
There are several kinds of operational constraint that influence the control of yaw in a helicopter:
both in cruising flight and in hovering flight, it is necessary for the neutral position of the pedals to be more or less centered; this improves pilot comfort by avoiding any need for the pilot to accept a continuous static offset in foot position;
the control available to the pilot via the pedals must provide sufficient maneuverability about the yaw axis, particularly while hovering;
the control available to the pilot via the pedals must provide sufficient margin to counter cross-wind (with a strong cross-wind, the yaw control is used to a considerable extent in order to maintain heading); and
control sensitivity, i.e. the ratio between movement of the pedals and the control applied to the tail rotor, must be optimized and must not increase excessively if protection is to be provided against the risk of piloted pumping.
Furthermore, on a helicopter, the equilibrium position for a tail rotor is not the same while cruising and while hovering.
Prepositioning the tail control via the anti-torque function is thus the result of a difficult compromise between the above-defined constraints.
In order to combine the mechanical constraints (pedal stroke) with the control sensitivity aspects, the pilot control cannot cover all desired positions: cruising and hovering (with the associated margins). After the constraints have been analyzed, a final pedal stroke is obtained that determines a compromise between ergonomics (cockpit design) and control sensitivity constraints (the risk of piloted pumping).
The compromises used in present configurations generally do not provide best optimization of all of the above-mentioned constraints and they generally give rise to the appearance of operational limitations. If the diagram is optimized for cruising, then it will be offset in static manner while hovering, with the consequence of an uncomfortable pilot position (feet offset), and consequently a limit on cross-wind coming from the side where the feet are statically offset. If it is desired to retrieve a control margin (ability to compensate for a cross-wind) by increasing the range over which the pedals provides control, then problems are rapidly encountered with piloted pumping associated with sensitivity that is too high. FIG. 2 shows the difference between a collective pitch and yaw diagram that is optimized for cruising, shown in dashed lines, and a diagram that is optimized for hovering, shown in continuous lines.